In most computer use of magnetic tape, two or more tape date channels are read or sensed by timing signals derived from a timing channel or from timing information which is incorporated in the data channel or channels. In the ideal situation, the electromagnetic head gaps which are used to magnetize the tape are in a straight line and precisely oriented perpendicular to the direction of tape travel. Departures from perpendicularity are known as skew, statically measured as an angle, dynamically as a time displacement.
Skew becomes important when the skew angle at the time of writing is different from the angle at time of reading. The general case of this is when one attempts to read tapes on a magnetic tape mechanism other than the one on which it is written.
If the reference tape is recorded on a magnetic tape unit having zero skew (i.e. determined, for example, by some precise optical method) then the reference tape may be used as a master standard to adjust any similar tape unit for skew, thereby ensuring the interchangeability of all tapes recorded or played back on tape units so adjusted.
Where the reference tape is recorded on a magnetic tape unit of unknown skew (relative to a hypothetical or actual zero skew master) it may still be used to adjust similar units, but in this case interchangeability can only be ensured within the limited class of units so adjusted.